Electrochemical cells are commonly used in a fuel cell configuration to produce electrical energy from reducing and oxidant fluid streams, or in an electrolysis cell configuration to produce product gases from a supply fluid such as producing hydrogen and oxygen gas from water, as is well known. Typical applications employ a plurality of planar cells arranged in a stack surrounded by an electrically insulating frame that defines manifolds for directing flow of reactant and product fluids. Electrochemical cells typically include an anode electrode and a cathode electrode separated by an electrolyte. In both fuel cells and electrolysis cells, operating efficiencies are enhanced by increased water permeability of a proton exchange membrane electrolyte.
For example, in a fuel cell configuration, it is common and well known to utilize a proton exchange membrane (“PEM”) as the electrolyte. Protons formed at the anode electrode move through the electrolyte to the cathode electrode, and it is generally understood that for each proton moving from the anode side to the cathode side of the electrolyte, approximately three molecules of water are dragged with the proton to the cathode side of the electrolyte. To prevent dry-out of the PEM, that dragged water must be replaced or returned to the anode side of the PEM by osmotic flow. Osmotic flow requires that the water content at the anode side of the PEM be less than at the cathode side to provide the required driving force. Additionally, during operation of the fuel cell, water is produced (“product water”) at the cathode catalyst, and that product water must be removed by flowing it to either the anode side through the PEM, through a water transport plate in fluid communication with the cathode catalyst, or by entrainment or evaporation within the process oxidant stream passing by the cathode catalyst. Therefore, significant hydraulic pressure is required to remove the product water, especially at peak current densities of 2 amps per square centimeter (“ASCM”) or greater expected for fuel cells utilized in automobiles.
It is critical that a proper water balance be maintained between a rate at which water is removed from the cathode electrode and at which liquid water is supplied to the anode electrode. If insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing a rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reactant fluid leading to local over heating. Dry-out of the PEM electrolyte also results in degradation of the PEM electrolyte, as is known. Similarly if insufficient product water is removed from the cathode electrode, the cathode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow.
Many approaches have been undertaken to enhance water transport of an electrochemical cell, including efforts to increase water permeability of the PEM. Those efforts include decreasing a thickness of the PEM, such as by production of an ultra-thin integral composite membrane disclosed in U.S. Pat. No. 5,547,551 to Bahar et al., that issued on Aug. 20, 1996, and U.S. Pat. No. 5,599,614 that also issued to Bahar et al. on Feb. 4, 1997. While ultra-thin PEM electrolytes have enhanced water permeability, nonetheless, significant electrochemical cell performance limits result from restricted PEM water permeability. For example, localized membrane degradation is known to occur due to dry-out of the PEM at reactant inlets of a fuel cell. Additionally, fuel cell durability and performance is known to be degraded as a result of catalyst flooding with product water. Accordingly, there is a need for a proton exchange membrane with increased water permeability.